1. Field of the Invention
The present invention relates to a non-aqueous secondary battery capable of exhibiting excellent charge/discharge cycle characteristics even when being charged to a high voltage.
2. Description of Related Art
With the development of portable electronic devices such as portable phones and notebook personal computers and the commercialization of electric vehicles in recent years, there are needs for small, lightweight and high-capacity secondary batteries. As high-capacity secondary batteries that can satisfy such needs at present, non-aqueous secondary batteries (lithium-ion secondary batteries) using a lithium-containing composite oxide such as LiCoO2 as a positive electrode active material and using a carbon material as a negative electrode active material have been introduced to the market. And as devices to which non-aqueous secondary batteries are applied are making further advancement, a larger capacity and larger energy density are required of non-aqueous secondary batteries, for example.
To increase the energy density of a battery, a high-capacity positive electrode active material or positive electrode active material capable of functioning at a high potential may be used. From the viewpoint of the latter, lithium-cobalt oxides with increased final voltage and spinel lithium-manganese oxides capable of functioning at a high potential have been studied at present.
For example, although LiCoO2 is generally used in a battery to be charged at a voltage of 4.3V or less against lithium, it has been reported that a battery using an oxide obtained from the partial replacement of Co of LiCoO2 with other metal element can be charged/discharged at a voltage of 4.4V or higher. Further, it has been found that a battery using the lithium-containing composite oxide represented by the general formula LiNixMyMn2-x-yO4 (where M is at least one transition metal element other than Ni and Mn, and x satisfies 0.4≦x≦0.6 and y satisfies 0≦y≦0.1) can function at a potential of 4.5V or higher against lithium (e.g., JP 9-147867 A and JP 11-73962 A).
However, when the lithium-containing composite oxide represented by the general formula LiNixMyMn2-x-yO4 discussed above or other positive electrode active material is used to form a battery, and the battery is charged at a high voltage, the positive electrode active material reacts with a non-aqueous electrolyte, which may lead to the deterioration of the charge/discharge cycle characteristics of the battery. Such deterioration of the charge/discharge cycle characteristics is more likely to occur when the battery is charged to 4.4V or higher against lithium, it is even more likely to occur when the battery is charged to 4.5V or higher against lithium, and the deterioration becomes particularly significant when the battery is charged to 5V or higher against lithium.
With the foregoing in mind, the present invention provides a non-aqueous secondary battery capable of exhibiting excellent charge/discharge cycle characteristics even when being charged at a high voltage.